Dynamic vision sensors (DVS) are new integrated circuits of the video camera variety, although they are not specifically such. In commercial video cameras, the apparatus records photogram after photogram. In DVS, there are no photograms. The integrated circuit contains a photo sensor matrix, similar to video cameras. In video cameras, each photo sensor is sampled with a fixed frequency. However, in DVS, the pixels are not sampled. Every pixel calculates the time derivative of the light it senses, and when this exceeds a certain level (threshold), the pixel emits an “event” outwards. The event usually consists of the (x,y) coordinate of the pixel within the two-dimensional photo sensor matrix. In this way, the output of a DVS consists of a flow of (x,y) coordinates of the various pixels that detect a change in the intensity they sense. This type of DVS sensor were reported for the first time by Lichtsteiner, Delbruck and Posch in 2006 (“A 128×128 120 dB 30 mW Asynchronous Vision Sensor that Responds to Relative Intensity Change” in Visuals Supplement to ISSCC Dig. Of Tech. Papers, San Francisco, 2006, vol., pp 508-509 (27.9) and subsequently in more detail in P. Lichtsteiner, C. Posch and T. Delbruck, (“A 128×128 120 dB 15 μs Latency Asynchronous Temporal Contrast Vision Sensor”, IEEE J. Solid-State Circuits, vol. 43, No. 2, pp. 566-576, February 2008).
More recently, Posch has reported a new prototype (C. Posch, D. Matolin and R. Wohlgenannt, “A QGVA 143 dB dynamic range asynchronous address-event PWM dynamic image sensor with lossless pixel level video-compression”, Solid-State Circuits, 2010 IEEE International Conference ISSCC, Dig of Tech Paper, pp. 400-401, February 2010).
However, in these DVS sensors, the photocurrent Iph sensed by a photo sensor is firstly transformed into voltage by means of a logarithmic conversion. This voltage is firstly amplified and its time derivative subsequently calculated. A crucial parameter is the voltage gain in this first amplification. The greater the amplification, the more sensitive the sensor will be to the “Temporal Contrast”. The problem is that this amplification should be carried out within each pixel of the matrix and should be carried out by a circuit which consumes little power and little area in the microchip. Moreover, it is important that it is carried out by a circuit which does not undergo too much mismatch in the gain value from one pixel to another, given that in the contrary case, it would introduce much variation into the behavior of the various pixels in comparison to one another, thereby reducing the overall sensitivity of the sensor. The DVS reported to date employ voltage amplification stages based on circuits with capacitors. In integrated analog circuits, the capacitors have low-mismatch between one another and are therefore highly suitable for carrying out voltage amplification stages. However, in DVS, obtaining voltage gains of around 20 to 100 (or over) is desirable. Upon doing so with capacitors, at least two capacitors are required, the value proportion of which is equal to that of the desired gain. Given that the area of the capacitors is proportional to their value this means that one of the capacitors should have an area which is between 20 and 100 times greater than the other. The end result is that a large part of the area of the pixel is consumed in the capacitors.
A possible alternative may be to obtain the voltage gain by means of two consecutive stages, given that the gain of each stage is multiplied. However, the synchronisation required between the two consecutive stages also makes it too long, thus reducing the speed of the DVS dramatically.